1. Field of the Invention
The present invention relates generally to magnetic encoders for automatic apparatus, and more particularly to such a magnetic encoder which is highly precise and suitable for use with a power source comprising a battery. This invention may be practiced in the form of a rotary type encoder or a linear type encoder.
2. Description of the Prior Art
Encoders are widely used in an automatic apparatus for positioning a movable part of the apparatus. The encoders measure a moving distance of the part in terms of an angle of rotation of a shaft of a motor, for example, and translate the measured angle into an electric code signal.
The encoders are classified into rotary type encoders and linear type encoders. The rotary type encoders are further classified into the incremental type measuring the number of pulses produced upon rotation, and the absolute type reading recorded codes from a rotor. In terms of the detecting method, the encoders are divided into the optical type and the magnetic type. In recent years, the incremental type magnetic encoder are widely used in view of their low manufacturing cost and high reliability.
One example of conventional rotary magnetic encoders is illustrated in FIG. 6 of the accompanying drawings. The illustrated encoder 1 includes a multipolar magnet rotor 3 having a row of alternate North and South poles 2N and 2S formed on the periphery of the magnet rotor 3 at a fine pole pitch, and a magnetoresistive element 5 (so-called MR sensor) disposed in confrontation to the periphery of the magnet rotor 3 with an air gap 4 defined therebetween. The magnet rotor 3 may be formed of a single magnetic body, or a rotor drum having a magnetic layer coated on its peripheral surface.
Each of the North and South poles 2N, 2S has a width .lambda. which is equal to an electric angle of 360.degree. or 2.pi. radian. The magnetoresistive element 5 is formed of a ferromagnetic material having the magnetoresistive effect.
The principle of the magnetic encoder of the foregoing construction will be described with reference to FIG. 7. The ferromagnetic magnetoresistive element 5 comprises a conductor (i.e., magnetoresistive member) 6 composed of a film of ferromagnetic material having an effect of magnetoresistance.
The conductor 6 formed a film of a strip of ferromagnetic metal such as nickel-cobalt having a thickness of several thousands .ANG. is formed on a base plate of glass by vacuum evaporation method or by an etching method.
When the conductor 6 is disposed over the multipolar magnetic body 3 as shown in FIG. 7, the magnetic flux extends from a North pole 2N and an adjacent South pole 2S in a direction perpendicular to the direction of an electric current flowing through the conductor 6. In this instance, the resistance of the conductor is caused to decrease by a magnetic flux 7X extending predominantly in a transverse direction of the conductor 6 as shown in FIG. 8. Designated by 7Y is a magnetic flux extending in a direction perpendicular to the plane of the conductor 6. The rate of change in the resistance of the conductor is several %.
If the conductor 6 is disposed at a phase .theta. with respect to the respective pole 2N or 2S having a width .lambda. equal to an electric angle of 360.degree. or 2.pi. radian, then a resistance R(.theta.) of the conductor is represented by the following equation: EQU R(.theta.)=R-.DELTA.r.multidot.cos .theta.
where R is a resistance of the conductor 6 at a position .lambda./4 or 3.lambda./4, .DELTA.r is a rate of change of the resistance.
The magnitude of the transverse magnetic flux 7X varies with the phase .theta., the conductor 6 and the width of the individual magnetic pole. The resistance R of the conductor 6 varies with the magnitude of the transverse magnetic flux 7.
In the case of the magnetoresistive element 5, as opposed to other types of magnetic sensor, an output signal from the magnetoresistive element 5 does not change at a center of the magnetic field (i.e., at the middle of the individual pole 2N, 2S) due to the absence of the transverse magnetic flux at the center of the magnetic field.
The above-mentioned magnetoresistive element 5 composed of a single conductor 6 is not practical. Accordingly, a conventionally used magnetoresistive element 5' as shown in FIG. 9 has two series connected conductors 6a, 6a' spaced from each other by a distance .lambda., each conductor 6a, 6a' being composed of two conductor portions connected at one end and folded into a U-shape.
The U-shaped conductors 6a, 6a' spaced by the width .lambda. of each pole are 180 degrees out of phase. An output terminal 12 is connected to a connecting point of the conductors 6a, 6a'. One end of the conductor 6a is connected to a current supply terminal 13 which in turn is connected to the positive side of a power source 14. One end of the conductor 6a' is connected to another current supply terminal 15 connected to the negative side of another power source 16. Output from the magnetoresistive element 5' is led out from output terminals 18-1, 18-2, the output terminal 18-1 being connected to the output terminal 12 while the output terminal 18-2 is connected to a connecting point 17 between the power sources 14, 16.
With the magnetoresistive element 5' thus constructed, the resistance of the conductors 6a, 6a' decreases in response to a component of the magnetic field extending parallel to the magnetic face of the magnet rotor 3.
This magnetic field component is maximum at a border between two adjacent poles of the magnetic rotor 3 and is equal to zero at the midpoint of each pole. As a consequence, the polarity of the conductors 6a, 6a' spaced by a distance .lambda./2 changes upon rotation of the magnet rotor 3, so that the number of rotation of the magnet rotor 3 can be measured by counting through the output terminals 18-1, 8-2 the number of times when the potential at the midpoint between the conductors 6a, 6a' becomes zero.
In the case of the magnetoresistive element 5' having the conductors 6a, 6a', a change in the potential at the midpoint between the conductors 6a, 6a' is indicated by an output signal wave form 22, 22' which tends to have a relatively small width. This tendency becomes marked as the spacing between the magnet rotor 3 and the magnetoresistive element 5' is reduced.
Since the output signal having such a narrow wave form includes a large component of nearly zero potential, a measurement of the zero potential points is likely to contain an error due to fluctuation of a reference voltage particularly when detected analog signals are converted into digital signals. The magnetoresistive element is likely to operate improperly when subjected to noise.
The magnetoresistive elements 5' does not produce a magnetic encoder signal of an A phase and a magnetic encoder signal of a B phase which is 90 degrees out of phase with respect to the A phase magnetic encoder signal. In order to obtain both of the A phase encoder signal and the B phase encoder signal, The magnetoresistive element 5' is combined with an additional magnetoresistive element 5' (not shown) disposed at a distance .lambda./4 from the first-mentioned magnetoresistive element 5'. The additional magnetoresistive element 5' includes two conductors (not shown but designated by 6b, 6b' for purposes of explanation) which correspond to the conductors 5a, 5a', respectively.
In the two magnetoresistive elements 5', 5', the conductors 6a, 6a' are used for producing an A phase encoder signal while the conductors 6b, 6b' are used for producing a B phase encoder signal. The conductors 6a, 6a' are 180 degrees out of phase with each other. Likewise, the conductors 6b, 6b' are 180 degrees out of phase. Thus, the conductors 6a, 6a' or 6b, 6b' are shifted or spaced by a distance equal to n.lambda.+.lambda./4 (n is an integer greater than or equal to 1).
The magnetic encoder signals produced from the magnetoresistive elements 5' are processed by a circuit shown in FIG. 11.
The signal processing circuit 8 for a magnetic encoder having the two magnetoresistive elements 5' includes four resistors 9-1, 9-2, 9-3, 9-4 each connected in series with one of the magnetoresistive elements 5' so as to form two bridging circuits for translate a change in the resistance into a change in the voltage, and two voltage comparators 10-1, 10-2 each connected with a corresponding one of the bridging circuits, thereby obtaining two encoder signals 11-1, 11-2 of rectangular wave forms which are 90 degrees out of phase, as shown in FIGS. 12(a) and 12(b).
An angle of rotation of the encoder can be obtained by counting the number of encoder signals 11-1, 11-2 of rectangular wave forms.
In the signal processing circuit 8 including the magnetoresistive elements 5' shown in FIG. 11, the output voltage appearing at the midpoint potential or the connecting point between the conductors 6a and 6a'(6b, 6b') is used as a magnetic encoder signal output.
The magnetic encoder having the magnetoresistive elements 5' described above has a drawback because the measurement is achieved in such a manner as to count respective portions of the output signal wave forms extending across the zero level. Such a measurement is likely to be negatively affected by fluctuation of a reference voltage particularly when the output wave forms are converted into digital signals. The measurement is susceptible to noise and the magnetoresistive element is likely to operate incorrectly with the result that precise magnetic encoder signals are difficult to obtain.
With the foregoing drawbacks in view, various attempts have been made by the present inventor and almost satisfactory results have been obtained by a magnetic encoder of a construction described below. The magnetic encoder includes a multipolar magnet rotor having a row of alternate North and South poles of equal widths, and a magnetoresistive element disposed in confronting relation to the periphery of the multipolar magnetic rotor. The magnetoresistive element includes a group of series connected conductors arranged in a comb-like shape and having the magnetoresistive effect, the conductor group being disposed over a width (2n+1).lambda. where n is an integer greater than or equal to 0 and .lambda. is a width of each individual magnetic pole. An output terminal is connected to the midpoint of the group of conductors for outputting a magnetic encoder signal. The encoder signal thus produced has a substantially rectangular wave form or a trapezoidal wave form.
Since the series of conductor groups having the magnetoresistive effect are arranged in side-by-side juxtaposition at a pitch equal to (2n+1).lambda., the magnetoresistive element has a relative small area and hence is compact as a whole, and can be manufactured at a low cost.
FIG. 13 illustrates a magnetoresistive element 19 used in a magnetic encoder. Illustrated magnetoresistive element 19 includes a magnetoresistive element member 19A for producing a magnetic encoder signal for an A phase and a magnetoresistive element member 19b for producing a magnetic encoder signal for a B phase.
The magnetoresistive element 19 is so constructed as to produce an A phase magnetic encoder signal and a B phase magnetic encoder signal which is out of phase with respect to the A phase magnetic encoder signal by n+.lambda./4 where n is an integer greater than or equal to 1 and given independently of an integer n described later on, and .lambda. is a width of each magnetic pole of a multipolar magnetic body. In the illustrated embodiment, n=1 so that the B phase magnetic encoder signal is out of phase by .lambda.+.lambda./4 with respect to the A phase magnetic encoder signal. To this end, the magnetoresistive element member 19A for the A phase includes a group of series connected linear conductors having the magnetoresistive effect and arranged side-by-side into a comb-like shape. And, the magnetoresistive element member 19B for the B phase includes a group of series connected linear conductors having the magnetoresistive effect and arranged side-by-side into a comb-like shape. The B phase magnetoresistive element member 19B is disposed out of phase by n.lambda.+.lambda./4 with respect to the A phase magnetoresistive element member 19A where n is an integer greater than or equal to 1. In the illustrated embodiment, n=1, so that the B phase magnetoresistive element member 18B is out of phase by by .lambda.+.lambda./4 with respect to the A phase magnetoresistive element member 19A. The magnetoresistive element members 19A, 19B are formed on a base plate 25 of glass by the vacuum evaporation method or the etching method described above.
The A phase magnetoresistive element member 19A and the B phase magnetoresistive element 19B are preferably deposited on the base plate 25 by the vacuum evaporation method so as to form a single magnetoresistive element. Alternatively, the magnetoresistive elements 19A and 19B may be formed on different base plates, then they are disposed n.lambda.+.lambda./4 out of phase with each other, thereby forming the magnetoresistive element 19.
The magnetoresistive element 19 may include two or more pairs of the A phase magnetoresistive element member 19A and the B phase magnetoresistive element member 19B, regardless of whether or not they have a common base plate. The magnetoresistive element member 19A constituting a part of the magnetoresistive element 19 is composed of a group of series connected conductors arranged side-by-side in a comb-like shape, the conductors 20 being disposed over a magnetic pole width (2n+1).lambda. where n is an integer greater than or equal to zero and .lambda. is a width of each individual magnetic pole. If n=0, then the conductor group 20 is disposed over a single magnetic pole width. An output terminal 12A is connected to one end of a conductor 20' which is disposed at the middle of the conductor group 20 as viewed from the direction of rotation of the magnet rotor 3. The output terminal 12A divides the conductor group 20 into two subgroups of conductors. The left conductor subgroup formed over a width .lambda./2 constitutes a magnetoresistive element member 21A while the right conductor subgroup formed over a width .lambda./2 constitutes a magnetoresistive element member 21A'.
The magnetoresistive element member 19B constituting a part of the magnetoresistive element 19 is composed of a group of series connected conductors arranged side-by-side in a comb-like shape, the conductors 20 being disposed over a magnetic pole width (2n+1).lambda. where n is an integer greater than zero and .lambda. is a width of each individual magnetic pole. If n=0, then the conductor group 20 is disposed over a single magnetic pole width. An output terminal 12A is connected to one end of a conductor 20' which is disposed at the middle of the conductor group 20 as viewed from the direction of rotation of the magnet rotor 3. The output terminal 12A divides the conductor group 20 into two subgroups of conductors. The left conductor subgroup formed over a width .lambda./2 constitutes a magnetoresistive element member 21B while the right conductor subgroup formed over a width .lambda./2 constitutes a magnetoresistive element member 21B'.
With this arrangement, the magnetoresistive element member 19A composed of above-mentioned two magnetoresistive element members 21A, 21A' and the magnetoresistive element member 19B composed of the above-mentioned two magnetoresistive elements 21B, 21B' are out of phase with each other by n.lambda.+.lambda./4.
The magnetoresistive element members 21A and 21A' are 180 degrees out of phase with each other, while the magnetoresistive element members 21B and 21B' are 180 degrees out of phase with each other.
One end of the last conductor of the magnetoresistive element member 21A is connected to one end of the first conductor of the magnetoresistive element member 21A'. Accordingly, the magnetoresistive element members 21A and 21A' are connected in series with each other. The output terminal 12A is connected to a connecting point between the two magnetoresistive element members 21A, 21A'. One end of the first conductor of the magnetoresistive element member 21A is connected to a current supply terminal 13A which is in turn connected to the positive side of a power source such as a battery 14A. One end of the last conductor of the magnetoresistive element member 21A' is connected to a current supply terminal 15A which in turn is connected to the negative side of a power source such as a battery 16A. An output terminal 18A-1 is connected to the output terminal 12A and another output terminal 18A-2 is connected to a connecting point 17A between the negative side of the power battery 14A and the positive side of the power battery 16A. An A phase magnetic encoder output appears between the output terminals 18A-1 and 18A-2.
Likewise, one end of the last conductor of the magnetoresistive element member 21B is connected to one end of the first conductor of the magnetoresistive element member 21B'. Accordingly, the magnetoresistive element members 21B and 21B' are connected in series with each other. The output terminal 12B is connected to a connecting point between the two magnetoresistive element members 21B, 21B'. One end of the first conductor of the magnetoresistive element member 21B is connected to a current supply terminal 13B which is in turn connected to the positive side of a power source such as a battery 14B. One end of the last conductor of the magnetoresistive element member 21B' is connected to a current supply terminal 15B which in turn is connected to the negative side of a power source such as a battery 16B. An output terminal 18B-1 is connected to the output terminal 12B and another output terminal 18B-2 is connected to a connecting point 17B between the negative side of the power battery 14B and the positive side of the power battery 16B. A B phase magnetic encoder output appears between the output terminals 18B-1 and 18B-2.
Since the conductor groups 20 of the magnetoresistive element 19 have the magnetoresistive effect, the resistance of the conductors 20 decreases in response to a component of the magnetic field acting in a direction parallel to the magnetic poles of a magnet rotor (identical to the magnet rotor 3 shown in FIG. 8).
The strength of this magnetic component is maximum at the border between two adjacent magnetic poles and is minimum or zero at the center of each individual magnetic pole, so that the magnetoresistive element members 19A and 19B each disposed over a range (2n+1).lambda. changes the polarity upon rotation of the magnet rotor 3. Accordingly, the number of rotations of the magnet rotor 3 can be measured by counting the number of zero potential portions of an encoder output signal appearing between the output terminals 18A-1 and 18A-20 and the number of zero potential portions of an encoder output signal appearing between the output terminals 18B-1 and 18B-2.
According to the magnetoresistive element 19, each of the magnetoresistive element members 19A, 19B is composed of two magnetoresistive element members 21A, 21A' or 21B, 21B' and includes a plurality of conductors 20 extending over a width n+.lambda./2 where n is an integer greater than or equal to 0. Accordingly, two output signals obtained from the respective midpoints 20' of the magnetoresistive element members 19A, 19B are each composed of a group of signals having a small width which are out of phase and overlapped with each other within a range n+.lambda./2, each signal being identical to the output signal shown in FIG. 12(a) or FIG. 12(b).
In reality, the group of signal wave forms are integrated wave forms and the output signals appearing between the output terminals 18A-1, 18A-2 and 18B-1, 18B-2 have a trapezoidal wave form or a rectangular wave form.
Those portions of the thus obtained output signal wave forms which extend near the zero potential level are vary small as compared to the output signal wave forms shown in FIG. 10 and hence the output signal wave forms includes a small number of portions extending across the zero potential level. Accordingly, a measurement at the zero point is unlikely to contain an error even when a reference voltage fluctuates. Furthermore, the output signals are not susceptible to noise and hence the magnetoresistive element 19 can operate properly.
If the two encoder signal wave forms produced respectively from the magnetoresistive element member 19A for the A phase and the magnetoresistive element member 19B for the B phase which is shifted in phase by n.lambda.+.lambda./4 with the magnetoresistive element member 19A are processed by the magnetic encoder signal processing circuit 8 shown in FIG. 11, then there are obtained two encoder signals 11-1, 11-2 as shown in FIG. 12(a) and FIG. 12(b), respectively. The encoder signals have different rectangular wave forms which are 90 degrees out of phase with each other. Therefore, by counting the encoder signals 11-1, 11-2 of the rectangular wave form, an angle of rotation of a magnetic encoder can be measured.
The incremental type magnetic encoder using the foregoing magnetoresistive element 19 is highly useful. However, the magnetoresistive element 19 is relatively large in size because two magnetoresistive elements 19A, 19B are disposed at a distance n.lambda.+.lambda./4 where n is an integer greater than or equal to zero, for producing an A phase encoder signal and a B phase encoder signal. The magnetic encoder having such relatively large magnetoresistive element 19 is large in size and costly to manufacture.
With the foregoing drawbacks in view, a further attempt has been made by the present inventor to devise a magnetic encoder having a modified magnetoresistive element, described later on.
As shown in FIGS. 14 and 15, the modified magnetoresistive element 19' is to constructed as to produce an A phase encoder signal and a B phase encoder signal which is .lambda./4 out of phase with the A phase encoder signal (.lambda. is a width of each magnetic pole of a multipolar magnetic body). To this end, the magnetoresistive element 19' includes an A phase magnetoresistive element member 19A' and a B phase magnetoresistive element member 19B' which are overlapped in .lambda./4 out of phase relation with each other.
The B phase magnetoresistive element member 19B' is formed on an insulating base plate 26 of glass by a suitable method such as the vacuum evaporation method or the etching method, the base plate 26 being wider than the width of the magnetoresistive element member 19B'.
An insulating film 27 is laid over an upper surface of the B phase magnetoresistive element member 19B' for protecting the the magnetoresistive element member 19B'. The insulating film 27 having substantially the same size as the base plate 26.
The A phase magnetoresistive element member 19A' is formed on an upper surface of the insulating film 27 by a suitable method such as the vacuum evaporation or the etching.
The upper surface of the A phase magnetoresistive element member 19A' is covered with a protective insulating film 28. The insulating film 29 having substantially the same size as the insulating film 26 stated above.
The insulating films 27, 28 have a plurality (six in the illustrated embodiment) of cutout recesses 27a, 28a (FIG. 15) along one edge thereof so as to expose output terminals 12A, 12B and current supply terminals 13A, 13B, 15A, 15B. The recesses 27a in the insulating film 27 are staggered with the recesses 28a in the insulating film 28. The terminals 12A, 12B, 13A, 13B, 15A, 15B have respective widths and positions such that the upper terminals 12A, 13A, 15A do not overlap the lower terminals 12B, 13B, 15B, respectively.
The A phase magnetoresistive element member 19A' and the B phase magnetoresistive element member 19B' of the magnetoresistive element 19' have the same construction as those of the magnetoresistive element 19 described above with reference to FIG. 13 and hence a description is no longer necessary.
A magnetic encoder incorporating the magnetoresistive element 19' produces an encoder signal of the same preciseness as the encoder signal produced by the encoder having the magnetoresistive element 19. Further, since the magnetoresistive element members 19A' and 19B' are overlapped, as opposed to the side-by-side arrangement of the magnetoresistive element members 19A, 19B, the magnetoresistive element 19' is smaller in width than the magnetoresistive element 19 (actually, the width is cut down by half or more). Accordingly, the encoder having the magnetoresistive element 19' is compact in size and can be manufactured less costly.
The encoder using the magnetoresistive element 19' is highly useful as described above, however, it has drawbacks described below.
Since the A phase magnetoresistive element member 19A' and the B phase magnetoresistive element member 19A' are overlapped in .lambda./4 out of phase relation to one another, they have different air gaps and, therefore, different sensitivities. The difference in sensitivity exerts great influence on the output level of the magnetoresistive element 19. For example, it create a difference in the level of the output wave forms. As a consequence, a separate output level wave form correction means is needed, which increase the manufacturing cost of the magnetic encoder. The magnetoresistive element 19 is not well suited for mass production because the overlapping arrangement of the two magnetoresistive element members 19A', 19B' rises the production cost.